1. Field of the Invention
The present invention relates to an object (a product including a machine, a manufacture, and a composition of matter) and a method (a process including a simple method and a production method). In particular, one embodiment of the present invention relates to a power storage device, a power storage system, a semiconductor device, a display device, a light-emitting device, another electrical device, a manufacturing method thereof, or a driving method thereof. In particular, one embodiment of the present invention relates to a power storage device and a method for charging the power storage device.
2. Description of the Related Art
A variety of power storage devices, for example, non-aqueous secondary batteries such as lithium-ion secondary batteries, lithium-ion capacitors, and air cells have been actively developed in recent years. In particular, demand for lithium-ion secondary batteries with high output and high energy density has rapidly grown with the development of the semiconductor industry, for electrical devices, for example, portable information terminals such as mobile phones, smartphones, and laptop personal computers, portable music players, and digital cameras; medical equipment; next-generation clean energy vehicles such as hybrid electric vehicles (HEVs), electric vehicles (EVs), and plug-in hybrid electric vehicles (PHEVs); and the like. The lithium-ion secondary batteries are essential as rechargeable energy supply sources for today's information society.
A lithium-ion secondary battery, which is one of nonaqueous secondary batteries and widely used because of its high energy density, includes a positive electrode including an active material such as lithium cobalt oxide (LiCoO2) or lithium iron phosphate (LiFePO4), a negative electrode formed of a carbon material such as graphite capable of occlusion and release of lithium ions, a nonaqueous electrolyte solution which consists of a lithium salt such as LiBF4 or LiPF6 dissolved in an organic solvent such as ethylene carbonate or diethyl carbonate, et cetera. The lithium-ion secondary battery are charged and discharged in such a way that lithium ions in the secondary battery move between the positive electrode and the negative electrode through the nonaqueous electrolyte solution and inserted into or extracted from the active materials of the positive electrode and the negative electrode.
The capacity of such a lithium-ion secondary battery and the like is determined by the amount of lithium inserted and extracted into/from the positive electrode. On the other hand, since decomposition reaction of the electrolyte solution occurs at the negative electrode, lithium is used in formation of a side reaction product (also referred to as solid electrolyte interphase (SEI)) et cetera, leading to a decrease in the capacity of the battery.
If decomposition reaction of the electrolyte solution which occurs at the negative electrode occurs also at the positive electrode, the electron transfer can be canceled out. However, the amount of reduction decomposition reaction at the negative electrode is larger than the amount of oxidation decomposition reaction at the positive electrode because the potential of the positive electrode is not sufficiently higher than the oxidation potential of the electrolyte solution.